Electrocardiogram is one of the most common medical examinations of cardiac electrical activity, which is performed by a medical professional or paramedics. Registration of ECG is relatively simple, however, its analysis requires a highly qualified physician with substantial experience in electrocardiography.
In general, there are two types of ECG tests, a one-time recording during a few seconds and a long-term monitoring which can be performed during various physiological tests, regular daily activities or as a round-the-clock monitoring in patients with serious medical disturbances. Each test requires a specialized protocol for registering and analyzing ECG signals.
One-time ECG recording is usually performed by ECG technicians or paramedics. The recording then is transferred to a physician for analysis, which includes a number of procedures. First, the cardiac complexes are visually identified by their characteristic shape consisting of a sequence of the following waves: P-, Q-, R-, S-, T- and sometimes U-wave. Next, these complexes are classified according to their origin as normal or sinus, supraventricular, ventricular complexes and their subtypes. The distance between two consecutive complexes is measured to determine the heart rate. Next, a number of the most important parameters including the amplitudes of each wave, the duration of PQ, QRS, and QT-intervals, and the amplitude of ST-segment are measured. Finally, the signals are compared with the recordings that were previously obtained from the same patients to determine serial changes in cardiac electrical activity.
Comparison of serial recordings is an important part of standard ECG examination that allows detection of changes and determining their time course. The comparison is performed visually by an experienced medical professional. The accuracy of this subjective comparison is not high and varies among physicians. The accuracy is not stable even in the same physician when the same measurements are repeated several times.
There are a number of prior art computerized systems that follow these basic steps of analysis and measure characteristic waves of ECG and prepare preliminary report for physicians. Since the number of analyzed variables and their combinations is large, these systems use sophisticated processing algorithms that require fast and powerful microprocessors or computers with a large memory available for processing.
Systems for long-term monitoring consist of two types, recording and real-time systems. Recording systems include 24-hour Holter monitors and event monitors, which record the data after a manual signal (event). Processing of these recordings, which include a large amount of data, consists of computer-assisted scanning with subsequent manual verification by an experienced medical professional. The results of analysis which include average heart rates, number of normal and types of abnormal beats during different periods of time, are submitted to a physician for final verification and conclusion.
Real-time systems include event-monitors, bedside monitors, stress-test systems and other devices for monitoring 1-2 critically important parameters and generating alarm or presenting the output information on a monitor. These systems perform an incomplete examination tracking the changes in heart rate and sometimes changes in the ST-segment. While this information is important for real-time control of a test or treatment, a number of important ECG changes, including changes in Q-, T-, or P-wave amplitude, QT-duration, are not exposed by this analysis.
It is known to provide portable ECG monitors that will sound an alarm or other signal to alert the user or an attendant of abnormal or unusual changes in the waveforms of the ECG signal. Such devices are, for example, disclosed in U.S. Pat. Nos. 4,193,393; 4,679,144; 5,033,475; 5,501,229 and 5,724,983. A system is also known, from U.S. Pat. No. 6,038,469, that includes at least one monitoring module for receiving ECG signals, a circuit for analyzing the signal, a plurality of parameters related to a patient's ischemic condition, and a network for exchanging data with a central unit, either by hard wire or telemetry. The monitor can be used in an ambulatory application in which the ECG signals are recorded and later sent to a central processing unit or units, which may be capable of sending information and data to the portable unit(s).
Shusterman et al. U.S. Pat. No. 5,967,995 has identified small cumulative changes in the series of cardiac inter-beat intervals using the Principal Component Analysis (PCA). This method accurately identified unstable dynamics of cardiac rhythm and predicted cardiac arrhythmias as early as several hours before the event when all known physiological indicators remained normal. The Shusterman et al. invention further extends the applications of PCA to the ECG signal.